The invention lies in the printing technology field. More specifically, the invention relates to a method for the combined printing of a print carrier or stock with two ink systems, and to a printing machine of hybrid construction having a flexo or flexographic printing unit and an offset printing unit disposed downline therefrom, for performing the method.
In recent years, a trend towards labels embellished with metal effects has intensified. In order to produce a four-color printed image bordered with a gold color, paper sheets completely covered with vapor-deposited aluminum are used in label printing works. A four-color print image is then printed onto those sheets by means of an offset printing machine. The offset inks are not printed directly onto the aluminum layer. They are printed onto a covering white primer applied to the aluminum layer. In the region of the border, the covering white primer is left out, and the offset inks black, cyan and magenta are not printed either. In order to produce a gold-colored appearance of the border, only yellow offset ink is printed directly onto the aluminum layer in the region of the border.
The use of paper sheets on which aluminum has been vapor-deposited is unfavorable, however, both from a cost point of view and from an environmental point of view (recycling). Technological problems also occur at the works which process the labels when the labels are detached from their carriers. For example, the labels of reusable, recyclable bottles have to be detached from the bottles before they are refilled. For that purpose the bottles are immersed in baths of alkaline solution. Because of the comparatively low adhesion of the covering white primer to the aluminum layer, the covering white primer and, together with this, the offset inks are already detached from the aluminum layer before the label is detached from the bottle. The detached covering white and the detached offset inks block the filters of the label removing plant.
A printing machine having a number of offset printing units, a flexo printing unit upstream of the offset printing units and a varnishing unit downstream of the offset printing units is described and shown in European patent EP 0 620 115 B1 (FIG. 2). Using a configuration of this type it is theoretically possible to print the above-mentioned paper sheets on which aluminum has been vapor-deposited, in that the covering white basic coating is applied by means of the flexo printing unit and the four-color print is applied onto that by means of the offset printing units.
Moreover, U.S. Pat. No. 5,630,363 describes a method for the combined printing of a printing material according to the flexographic printing and offset printing principle. German published patent application DE 44 35 307 A1 describes a method for the embossing and subsequent printing of a printing material.
It is accordingly an object of the invention to provide a novel printing machine and an associated process, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which renders it possible to produce metal effects cost-effectively on the printing materials serving for the production of labels or packaging. It is a further object to provide a printing machine by means of which the method can be implemented in a technologically beneficial way.
With the foregoing and other objects in view there is provided, in accordance with the invention, a printing method, which comprises: printing a printing material in a combined printing process with two ink systems, and thereby
first printing onto the printing material an ink selected from the group of solvent-based inks and radiation-curing inks; and
subsequently printing onto the printing material at least one offset-typical ink with an offset printing process.
In accordance with an added feature of the invention, the product is first printed with a metallic ink before being forwarded to the offset printing unit(s).
In accordance with an additional feature of the invention, the first printing step comprises printing the printing material several times with inks selected from the group consisting of solvent-based and radiation-curing inks prior to printing with the offset-typical ink.
The method according to the invention for the combined printing of a printing material with two ink systems is distinguished by the fact that the printing material is first printed with an ink which can be dried by radiation or with a solvent-containing inkxe2x80x94especially in each case with such a metallic inkxe2x80x94and is then printed with an offset ink and preferably with a number of offset inks.
With the method according to the invention, it is possible to produce labels, folding boxes or the like decorated with metal effects without the use of printing-material sheets on which aluminum has been vapor-deposited. The solvent-based or radiation-curing ink does not need to be applied to the entire area of the printing material, nor overprinted with the offset-typical ink. Therefore, no covering white primer is required either, and the problems associated with this when the labels are being detached from labelled products are also eliminated.
In a refinement of the method which develops the method according to the invention and is advantageous with regard to the achievement of a high ink layer thickness on the printing material, before being printed with the offset ink or the offset inks, the printing material is first printed at least twice, one after another, with the solvent-based or radiation-curing ink. In this case, it is possible to print twice, one after another, with precisely the same solvent-based or the same radiation-curing ink. However, it is also possible for two different solvent-based or two different radiation-curing inks to be printed one after another. For example, in one case the two solvent-based inks and in the other case the two radiation-curing inks can each differ from each other slightly in terms of their viscosity, composition or pigments.
In a refinement of the method which is advantageous with regard to spot coating of the printing material, in order to print the printing material with the solvent-based or radiation-curing ink or with the solvent-based or radiation-curing inks, the flexographic printing principle is used in each case. The solvent-based or radiation-curing ink can therefore be placed on the printing material in a manner suitable for the printed image. In printed-image regions where the offset-typical ink is printed directly onto the printing material, the printing-material coating with the solvent-based or radiation-curing ink is left out.
In accordance with another feature of the invention, which is advantageous with regard to xe2x80x9cwet on dryxe2x80x9d printing, after being printed with the solvent-based ink or with the solvent-based inks, the printing material is air-dried. In the case of the printing of the printing material twice, one after another, using the solvent-based ink/inks, first intermediate air-drying directly following the first printing process, and second intermediate air-drying directly following the second printing process are advantageous.
In a refinement of the method which is likewise advantageous with regard to xe2x80x9cwet on dryxe2x80x9d printing, after being printed with the radiation-curing ink/inks, the printing material and, put more precisely, the ink printed on it, is dried by irradiation with ultraviolet light or electron bombardment. If the printing material is printed twice, one after another, with radiation-curing inks, first UV or electron irradiation directly following the first printing process, and second UV or electron irradiation directly following the second printing process are advantageous. In many applications, however, xe2x80x9cwet on wetxe2x80x9d printing of the two solvent-based inks or the two radiation-curing inks can also be carried out. In this case, no intermediate drying needs to be carried out between the first and second printing process using the solvent-based or radiation-curing ink, and it is sufficient for the intermediate drying to be carried out between the last printing of the printing material with the solvent-based or radiation-curing ink and the first printing of the printing material with the offset-typical ink.
In a refinement of the method which is advantageous with regard to the application of a transparent protective varnish to the printing material, after printing with the single offset ink or the last of a number of offset inks, the printing material is printed with a water-based ink especially an emulsified varnish. Water-based inks and varnishes are very environmentally friendly, so that a protective varnish covering the entire printed format is possible without damaging emissions.
In a refinement of the method which is advantageous with regard to the application of a spot varnish to the printing material, the printing of the printing material with the water-based ink or with the emulsified varnish is carried out using the flexographic printing principle. The relief print form used in flexographic printing and made of elastic polymer is raised only at the printing points. It is therefore possible to provide selected regions within the entire printed format with a decorative glossy varnish.
In a refinement of the method which is advantageous with regard to finishing the printing material, the printing material is perforated, stamped, fluted, embossed or the like before being printed for the first time with an offset ink. By means of this finishing, preceding the offset printing and dividing and/or deforming the printing material, the printed image produced by means of the offset printing is not destroyed. The offset-typical ink is transferred to the finished printing-material surface by a rubber blanket, which makes very good contact with a relief on the printing-material surface produced during finishing. In this way, the offset ink can be printed, with equally good print quality and area coverage, on the depressed and elevated embossed regions produced during embossing.
In a refinement of the method which is advantageous with regard to the finishing of the printing material with a fabric-like surface structure, for example a so-called linen appearance, the printed material is embossed before it is printed for the first time with an offset ink. The embossing may be located in that region of the printed image printed with the solvent-based or radiation-curing ink before the embossing, and/or in that region of the printed image printed with the offset ink(s) after the embossing. By means of finely structured and, for example, fluted, lined, dotted embossing, the said fabric-like surface structure can be produced. By means of embossing over a somewhat larger area, it is possible for an embossed text to be produced. If the solvent-based or radiation-curing ink printed onto the printing material before the embossing is a metallic pigment ink, a gold, silver or bronze embossed text may be produced by embossing into its imprint, which appears metallic.
If the offset printing ink(s) are radiation-curing and, for example UV-curing, the embossing can be carried out within the in-line process, following the printing with the offset printing ink(s), by means of an embossing unit downstream of the last offset printing unit.
The printing machine according to the invention of hybrid design, having a flexo printing unit and a downstream offset printing unit, is distinguished by the fact that a UV dryer, an electron-beam dryer or an air-stream dryer forms a constituent part of the flexo printing unit or a drying station separate from the flexo printing unit, which is arranged downstream of the flexo printing unit and upstream of the offset printing unit. It is preferable if the printing machine has two such flexo printing units or flexo printing unit/drying unit pairs in a tandem arrangement.
The printing machine according to the invention is very suitable for the efficient in-line implementation of the method according to the invention. The printed image produced by means of the flexo printing unit or a number of flexo printing units and consisting of the solvent-based or radiation-curing ink has already been dried as a result of the use of the dryer or the drying station when the printing material is printed by the offset printing unit. Contamination of the offset-typical ink printed by means of the offset printing unit as a result of any deposition of a solvent-based or radiation-curing and not yet completely dried metallic ink is thus ruled out absolutely.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a printing method and machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.